He traveled to Zaire in 1960, where he practiced medicine and ran the 1,500-bed Mama Yemo hospital in the capital city of Kinshasa, with the goal of getting health care into rural areas of the country. He served President Mobutu Sese Seko as his personal doctor and was chief physician of the nation's army.[4]

In the mid-1970s, the Ebola virus broke out at a missionary hospital in rural Yambuku, near the Ebola River. The disease, which was characterized by severe sore throat, rash, abdominal pain, and bleeding from multiple sites, had killed 11 of the 17 medical staff at the hospital, forcing it to close. Panic was in the air, with roads blocked, river traffic stopped, and commercial air travel restricted. The army would not enter the area and President Mobutu was said to have left the country and fled to France, in the face of fears that the disease could spread to others, as those infected with the disease tried to escape the center of the outbreak.[4]

On his way back home to the United States for a home leave, Dr. Close returned to Kinshasa from Geneva, discussing the issue on the flight with two physicians from the Centers for Disease Control (CDC), Joel G. Breman and Peter Piot, who discovered the Ebola virus. Back in Zaire, Close obtained airplanes and pilots from the Ministry of Health to transport medical equipment to the affected area, using his personal connection to President Mobutu to obtain the access he needed. Close coordinated efforts in the area, ensuring that medical supplies were directed to where they were most needed. After providing protective equipment for hospital workers, sterilizing medical supplies, and quarantining patients, the team was able to break the chain of transmission of the virus, with almost 90% of the 318 people infected left dead.[4]

Blood samples that Close had collected in the 1970s were used to investigate the progress of the AIDS pandemic, showing that the rate of infection had been stable at 0.8% by comparing new samples to the older ones that had been collected in Zaire, one of the few sets of historical specimens available to perform this analysis. This showed that HIV infection rates could have been stable in rural Africa before it spread throughout the world.[4]

Disillusioned with Mobutu's policies, Dr. Close left Zaire in 1977. He moved to Big Piney, Wyoming, where he became a country doctor, making his final house call a month before his death.[4] During the 1995 Ebola outbreak, he was a liaison between the CDC and the Zairian government.

Close wrote four books, which included chronicles of his experiences as a doctor in Zaire and Wyoming.

Dr. William Close died at age 84 on January 15, 2009, in his home in Big Piney due to a heart attack. He was survived by his wife, three daughters, two sons, nine grandchildren, and a twin brother.[4]